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Wildlife Diseases
Basic Principles &
Concepts
OutlineMotivations to study wildlife disease
Defining & quantifying disease
Epidemiological factors
Impacts of wildlife disease
Motivations to study Wildlife Disease
Human health
Livestock health
Game management
Rare species management
Economics
Human HealthZoonotics
West Nile VirusEncephalitisFever
Birds Mosquitoes Humans
Infection ImpactsIdentified in 19991000’s of human cases / year since 2003
Human Health Anthropozoonitics
Humans Wildlife
Spread to other primatesRespiratory virusesMeaslesInfluenza Parasites? Polio? Yaws
Livestock HealthBovine Tuberculosis
Wildlife reservoirsWhite – tailed deer (MI, USA)European badgers (UK)
Agricultural Impacts (Michigan)Loss of TB-free status Restrictions on tradeRequirements for monitoring
Game ManagementChronic Wasting Disease
Wisconsin’s GoalsMinimize the affected areaMinimize prevalence
Management – decrease deer populationDelineate CWD management zonesRestructure hunting seasons
Management – decrease disease spreadRestrict cervid baiting & feedingRegulate farmed cervid industry
Rare Species ManagementDevil Facial Tumor Disease
Contagious cancerFatal
WidespreadOccurs over ~ 60% of rangeLocal prevalence up to 83%
Large population declines64% on average Up to 95% locallyPotential for extinction within 20 years
EconomicsZoonotics $100’s M
Rabies: > $300M / yr in US on detection, prevention, controlSARS: Canada lost millions in tourism after 2003 casesWest Nile Virus: $20M in 2002 public health costs to Louisiana
Bovine Tuberculosis $15M / yr Lost from MI agriculture without TB free status
Chronic Wasting Disease $10’s MLost from hunting-related revenue and commerce in WI in 2003 hunting season
Anthrax $M’sLost from tourism as Chobe NP closed during 2004 outbreak, and other African parks lost valued animals
Wildlife Disease Study Requires Interdisciplinary Science
Human Medicine
Veterinary Medicine
Wildlife EcologyWildlife Disease
What is Wildlife Disease ?What is it?
How is it transmitted?
How do we quantify occurrence?
What is Wildlife Disease ?Disease
“Any impairment that interferes with or modifies the performance of normal functions, including responses to environmental factors such as nutrition, toxicants, and climate;infectious agents; inherent or congenital defects; or combinations of these factors” (Wobeser 2006)
Zoonosis (zoonotic disease)Infectious disease that is transmitted naturally between animals and humans
Endemic / EnzooticA disease that occurs with predictable regularity and rate in a given population / area / time frame
Epidemic / EpizooticA disease that is occurring in a species / place / time where it is unexpected, or at a higher rate than expected
Types of Disease
Herbicides, insecticides-Human-synthesized chemicalsLead, selenium-Human-distributed chemicalsCyanobacterial toxins-Naturally occurring chemicals
ChemicalsHeat, cold, traumaPhysical agentsMalnutrition, vitamin deficiencyNutritional deficienciesDiabetesMetabolic alterationsInbreeding depression, mutationsGenetic disorders
NON-INFECTIOUS
Nematodes, flukes-EndoparasitesFleas, ticks-Ectoparasites
MetazoaMalaria - plasmodiumProtozoaChytrid fungusFungiMycobacterium spp.BacteriaRabies virusViruses
INFECTIOUSFamiliar ExamplesDisease Agents
Modes of Disease Transmission
Horizontal Vertical
Direct Indirect
Close EnvironmentalContact Contamination
-Intermediate host-Vector
-Skin-to-skin-Aerosol / droplet-Secretions / excretions-Sexual activity-Contacting carcasses
-Air-borne-Water-Fomites-Transport host-Paratenic host
-Parent – offspring
Defining Disease CasesMortality (field observations)
CarcassesIs dead animal with disease killed by disease?How many dead exist for every carcass recovered?
Infected (field observations, laboratory tests)
Clinical signs Pre-clinical
Exposed (laboratory tests)
ImmuneRecoveredCarriers
Quantifying OccurrenceIndividual Measures
Parasite load• # parasites / host
Case – fatality rates• # deaths / # infected
Population MeasuresIncidence• # new cases / # in pop• over specified time period
Prevalence• # existing cases / # in pop
Incident cases
Mortality / Recovery
Prevalence
The Epidemiological Triangle
HOST
PATHOGEN ENVIRONMENT
Vect
or
Host TypesPrimary / Definitive
The host in which the agent undergoes replication
IntermediateHost in indirect transmission cycles in which the agent does not reproduce
ReservoirA species in which the agent is maintained, usually without developing disease, and may be passed to others
CarriersHarbors infectious agent, lacks clinical signs, but can transmit infection to others
Dead end hostsA species that can be infected with the agent acquired from outside sources, but does not maintain the agent
Host FactorsBiology
AgeSexGeneticsSocial statusBreeding status
Immune StatusSusceptibilityImmunity
ConditionNutritionStress
BehaviorContactsMovements
Pathogen FactorsInfectivity
Ability to enter host
Life CycleHost requirementsReproductive rates
PathogenicityMortality / morbidity
SurvivalPersistence in environment
Vector Factors
Life CycleHost requirementsReproductive ratesEnvironmental effects - climate
SurvivalPersistence in environment
Environment FactorsStressors
ClimateSeason
LocationHabitat qualityPopulation densityProximity to infected animals
ExposuresPathogen reservoirsToxins
Impacts of Wildlife DiseaseCellular injury
Individual productivity loss
Individual mortality
Population impacts
Alteration of ecological communities
Impacts of Wildlife Disease
Energy Intake
Energy Assimilated
Production StorageRespiration
Individual ImpactsIncreased energy demands
ThermoregulationInflammatory / immune responseAltered behavior
Decreased energy intakeBehavioral• Lethargy or hiding decrease foraging• Avoidance of certain foods / feeding areas• Time / energy devoted disease Physical• Lack of appetite• Reduced mobility / ability to forage• Decreased digestive efficiency
• Increased excretions
Energy Intake
Energy Assimilated
Individual ImpactsRespiration less efficient
Poor thermoregulationDecreased activity
Decreased reproductionAbortions Smaller littersDecreased ability / attempts to mateInjury to reproductive system / organsAbnormal young or infant mortality
Less energy for storageDecreased growthDelayed maturity
Energy Intake
Energy Assimilated
Production
Storage
Respiration
Individual ImpactsDirect mortality
Indirect mortalityPredationStarvationHypothermia Other diseasesAccidentsDepressed growth
Energy Intake
Energy Assimilated
Production
Storage
Respiration
Impacts of Wildlife DiseaseCellular injury
Individual productivity loss
Individual mortality
Population impacts
Alteration of ecological communities
“A disease becomes important to a population when it causes sufficient reproductive failure or mortality to reduce breeding numbers below what would otherwise occur.” (Newton 1998 from Wobeser p 143)
Population ImpactsLimitation
Disease is a factor prohibiting population increase
RegulationDisease holds population near carrying capacity if contact, transmission & infection increase when animals are more densely populated
Stochastic eventsSmall populations will be especially vulnerable to extinction from highly pathogenic disease, or increased susceptibility to environmental changes due to disease stress
Population ImpactsPopulation size
May be decreased by disease
Spatial distributionAnimals may avoid habitat with disease riskSegments of the population may be extirpated
Age structureDisease risk and survival may vary by age
Sex ratioDisease risk and survival may vary by sex
Life history traitsBreeding strategies and age at maturity may determine resilience of population to disease mortality
Population ImpactsPopulation size
ReproductionDirectly impacted by disease via decreased reproductive success of individuals
MortalityDirectly impacted by disease mortality and additive mortality due to increased susceptibility to starvation, predation, accidents, etc.
Additive vs compensatory mortality
Immigration & EmigrationMay be affected by changes in behavior and activity
Ecological ImpactsLocal population extinction
Species rarity / extinction
Lack of prey availability
Lack of predator regulation
Open niches filled by invasive species
Ecological ImpactsLynx – snowshoe hare cycles
Hare density increasesHare become nutrient limitedIncreased disease (helminthe parasites) Increase susceptibility to predationHare density decreasesLynx population declinesPredation pressure decreases
Ecological ImpactsHawaiian bird communities
Humans bring non-native birds & mosquitoesNon-native birds bring diseases• Avian pox• Avian malariaNative populations restricted to high elevations free of mosquitoesLowland forests free for expansion of non-native populations
Discussion Questions